Many types of artillery shells carry sub-munitions; and such sub-munitions are often arranged in stacks within the hollow main body of the shell. The U.S. 155 mm M864/M483 artillery round, for example, carries M42/M46 shaped-charged grenades in an array of stacked columns arranged in a circular pack around the inner periphery of the main body. The circular pack has seven grenades around the inner periphery and one grenade on the center line. German Patent Publication DE 3841-908-A is another representative of this type of artillery shell and shows the basic packing arrangement for the shaped-charge grenades. The grenade's cylindrical casing is designed for stacking by having a reduced diameter at the fuze end, which fits into the cone of the shaped-charge at the opposite open end and results in the open end of the casing being supported by the shoulder formed at the reduced diameter of the next grenade in the stack. The stacks are arranged radially around the longitudinal axis of the main body of the shell to form a circular pack, in this instance with six columns of grenades around the perimeter and one column on the center line. If the main body of the shell is viewed in a transverse section, the section will have six grenades evenly spaced around the perimeter and one grenade in the center.
The pack is held together by spacer bars between the perimeter stack grenades. The spacer bars are usually made in segments for each circular layer, with pins or similar connectors between the segments. The M864/M483 round adds to this configuration a plastic (polyethylene) sleeve around the center grenade to take up the gap caused by the size of the inner diameter of the 155 mm casing in relation to the diameter of the M42/M46 grenades.
Typically at least one of the spacer bars in a transverse section will have some mechanism for locking itself to the shell casing to prevent the grenade pack as a whole from rotating within the shell in reaction to the spin imparted by the rifling. In the above publication, for example, the spacer bar segments at one of the perimeter sides have a ridge to engage in a slot in the shell casing to prevent rotation of the pack as a whole.
Several other patent publications depict variations of grenades stacked in a radial array of columns inside an artillery shell. European Patent Publication 481-874-A discloses a grenade pack without a column of grenades in the center. Instead, a central spacer bar extends the length of the projectile and is attached to a piston in front of the grenade pack. The piston and bar bear the force of the ejection charge to detach the base and push out the grenade pack.
Spacer bars are often made of steel or heavy metal to achieve a mass distribution sufficient to impart stabilizing spin to the ejected grenades. The process of assembling the grenade pack with solid spacer bar segments requires the grenades to be assembled one layer at a time, with a press used to seat the spacer segments. In response to this loading difficulty, U.S. Pat. No. 5,473,988 discloses spacer bars made of nylon half-bar segments, each with a shallow groove pattern in the surface that cups the grenade, and slanted guiding surfaces that expand the width of the spacer when the half-bars are pressed together. The grooves compress against the grenade when the pack is pressed.
While the above configurations are intended to provide a tight pack and to prevent counter-rotation of the pack as a whole inside the shell, little or no importance has been given to the rotational orientation of the individual grenades within the stack or to the related problem of maintaining a particular orientation. Thus, while the spacer bar configuration prevent pack rotation, they do not prevent rotational movement of individual grenades within the pack.
There are pack configuration for special grenades that would prevent individual grenade rotation. German Patent Publication DE 3732-752-A shows a seven pack radial array with an eighth grenade in the center, with spacer bars between the radial grenades and a cylindrical sleeve around the center grenade. The grenades in this shell are a special configuration of grenade with three rods attached to the casing for the purpose of providing a stand-off of the shaped charge from an armored surface at detonation. Consequently, the spacer bars and sleeve must have grooves into which the rods can be fit. This special configuration of grenade and pack will prevent individual grenade rotation, but the same spacer bars and sleeve would not prevent rotation of grenades that do not have these stand-off rods.
As a result of proving grounds testing for safety certification of a grenade with a new fuze, the present invention determined that malfunction of slide-type fuzes such as the new fuze being tested can be related to the rotational orientation of the grenade in the pack; specifically, that the optimum orientation is to align the fuze slide deployment axis outward along the radial axis of the shell for those grenades in the perimeter columns of the pack. Merely making such alignment at the time of packing would be insufficient, however, if the grenades were not prevented from rotating within the pack to a different fuze orientation before in-flight ejection.
Efforts were made to prevent grenade rotation by using shims to compensate for variations in height of the grenade columns so that each column was subject to equal press force and compression, but hot and cold tactical vibration testing revealed that these initial compression forces were not retained. It was also realized that modifying the grenade casings to mechanically interlock with the spacer bars would not be cost effective and could compromise the performance of the grenades. An effective, low-cost solution to the problem was found in the replacement of the hard polyethylene sleeve around the center grenade by a larger diameter sleeve of more resilient material. Silicon rubber and soft polyurethane sleeves were tested and both found to prevent rotational migration.
The silicon rubber sleeve was selected for the safety certification of the new grenade fuze. Test 155 mm M864/M483 rounds were assembled with live M42/46 grenades equipped with the new fuze, and the grenades were packed with the fuze slide deployment axis of the perimeter grenades aligned radially outward. (The slide deployment axis of the center grenades will always be radially outward because those grenades are on the longitudinal axis or center line of the shell.) Over-sized resilient silicon rubber sleeves, formed by joining half-sleeves, were placed around the grenades in the center column to take up the gap between the center grenades and the perimeter grenades. The test rounds were subjected to a variety of environmental tests and eventual firing. No discernable rotational migration of the grenades was observed after the environmental tests. The munition was safety certified.